1. Field of the Invention
The present invention relates to an inverted Galilean finder that can be mounted in a small space and has a high magnification. The present invention relates also to a camera, especially a lens-fitted photo film unit, that is provided with such an inverted Galilean finder.
2. Background Arts
Lens-fitted photo film units are widely used as a kind of single-use economy cameras. The lens-fitted photo film unit, hereinafter referred to as the film unit, is preloaded with a roll of unexposed photo filmstrip, so the user can take photographs instantly at the purchase of it. After the completion of exposure, the user has only to forward the film unit to a photofinisher, to obtain the developed photo filmstrip and photo-prints made from the photographed pictures. Considering the above nature, being low-price and compact is important for the film unit.
As an optical finder of the film unit, the inverted Galilean finder consisting of a concave objective lens and a convex eyepiece has been used because of its simple structure and compactness. The Galilean finder provides comparatively good performances at a low cost.
However, because of the limit in the mounting space, the Galilean finders used in the conventional film units merely have a finder magnification of about 0.4 to 0.6, which is too small for many users. So it has been desired to provide the film unit with a finder having a larger magnification.
In order to enlarge the magnification of the finder, it is usual using larger objective and eyepiece lenses. But this solution has problems that it needs a larger mounting space and a higher material cost for the finder, so the compactness and cheapness of the film unit are deteriorated.
In view of the foregoing, an object of the present invention is to provide an inverted Galilean finder that has a high magnification, can be manufactured at a low cost and does not need a larger mounting space. The present invention also has an object to provide a lens-fitted photo film unit provided with such an inverted Galilean finder.
To achieve the above and other objects, an inverted Galilean finder of the present invention is composed of an objective lens of a negative power and an eyepiece of a positive power, the objective lens having a concave surface oriented to the eyepiece, the eyepiece having a convex surface oriented to the objective lens, wherein the inverted Galilean finder satisfies the following conditions:
xe2x88x920.75xe2x89xa6f1/f2xe2x89xa6xe2x88x920.60xe2x80x83xe2x80x83(1) 
f1xe2x89xa6xe2x88x9222xe2x80x83xe2x80x83(2) 
xe2x88x923.2xe2x89xa6f1/dxe2x89xa6xe2x88x921.5xe2x80x83xe2x80x83(3) 
wherein f1 is a focal length of the objective lens, f2 is a focal length of the eyepiece, and xe2x80x9cdxe2x80x9d is a surface distance between the concave surface of the objective lens and the convex surface of the eyepiece on an optical axis of the Galilean finder.
The inverted Galilean finder of the present invention provides a magnification of about 0.6 to 0.8. The objective lens and the eyepiece are preferably formed from a polystyrene resin by injection molding. Mounting the inverted Galilean finder satisfying the above conditions provides a compact camera or a lens-fitted photo film unit that has a sufficiently large finder magnification of about 0.6 and 0.8.
If the value f1/f2 is above the upper limit of the first condition (1), the finder magnification cannot be so high, so that the remarkably superior performances to the conventional finder is not achieved. Below the lower limit of the first condition, indeed the finder magnification becomes higher, it is necessary to enlarge the lens size. Thus, the mounting space for the finder would increase, deteriorating the compactness of the film unit.
The second condition: f1xe2x89xa6xe2x88x9222 defines an upper limit of the focal length of the objective lens having the negative power. When designing a lens system that satisfies the above first condition, power arrangement of the lens system is improved using an objective lens that satisfies the second condition, so it becomes easy to correct aberrations of the lens system. The second condition also makes it easy to define such an air space between the lenses that is suitable for use as a finder mounted in a lens-fitted photo film unit.
The third condition: xe2x88x923.2xe2x89xa6f1/dxe2x89xa6xe2x88x921.5 defines a condition relating to the air space between the concave surface of the objective lens and the convex surface of the eyepiece on the optical axis, on the premise that these lenses constitute a lens system that satisfies the above first and second conditions. Above the upper limit of the third condition, the surface distance xe2x80x9cdxe2x80x9d and thus the mounting space for the finder become so large that the lens-fitted photo film unit would loose the compactness. Below the lower limit of the third condition, the refractive power of the objective lens becomes so small that it would be necessary to enlarge the finder mounting space. Otherwise, the surface distance xe2x80x9cdxe2x80x9d becomes so small that it would be hard to design composition of the two lenses. In either case, it becomes disadvantageous for the lens system to serve as a finder.